As depicted in FIG. 1(a) conventional gas turbine combustion chambers 10 receive high pressure, high velocity air exiting from the compressor 20 of a gas turbine engine. (The air from the compressor 20 may exit via an Outlet Guide Vane 22.) This high pressure and high velocity air first enters a cavity 11 outside the combustion chamber 10. Most of this air then enters the combustion chamber 10 through the fuel injector 12, air admission ports and/or any cooling features, e.g. in the upstream end wall 14. A small remainder of the air also bypasses the combustion chamber 10 via passage 15. Some of this air in the bypass passage 15 may enter the combustion chamber via combustion chamber lining cooling ports 13 and the remainder may cool the turbine High Pressure Nozzle Guide Vanes 30 and/or any other turbine components.
In early combustion chambers, an example of which is shown in FIG. 1(b), the combustion chamber cowl 16 was extended forward into a snout 17 very close to the compressor exit. This snout 17 directs air into the combustion chamber 10 and allows the surplus air to pass into passage 15. By contrast, the later combustion chamber 10 shown in FIG. 1(a) has a smaller snout 17, although a diffuser 18 is provided at the compressor exit.
In both of the aforementioned examples, fuel is introduced directly into the combustion chamber via the fuel injector 12 where it is mixed with air and burnt in a single flame zone (per sector). In actuality some of the fuel burns immediately on meeting air in a “non-premixed” or “diffusion” flame mode. By contrast, in radially staged combustors, e.g. as shown in FIG. 1(c), the fuel is still sprayed directly into the combustion chamber 10 for mixing and burning, but two separate flame zones (per sector) inside the combustion chamber are defined. The first flame zone 19a is a pilot zone, whilst the second radially outer zone 19b is a main flame zone.
In order to optimise the performance of a conventional combustion chamber (whether radially staged or not) for emissions (Nitrogen oxides, e.g. NO and NO2, Carbon monoxide, un-burnt hydrocarbons), the fuel and air have to be rapidly mixed prior to combustion in order to set up a flame of the required air to fuel ratio (AFR) or stoichiometry. In lean systems the flame must only predominantly exist where the fuel air mixture has mixed to a lean AFR. This is in order to prevent the combustion of fuel rich pockets that would result in high Nitrogen Oxide (NOx) emissions. However, achieving adequate mixing to minimise NOx production whilst maintaining combustion efficiency and stability is a challenging task. Furthermore, achieving acceptable relight at altitude, weak extinction, soot emissions, pressure loss and traverse performance add to the challenge.
The present disclosure therefore seeks to address these issues.